Electrical transformers commonly transfer electrical energy from one electrical circuit to another electrical circuit through inductively coupled conductors or coils. Such transformers commonly include one or more conductive coils that are maintained in proximity to one another such that electrical power can be input through a primary or first coil and generates electrical output power in a secondary, adjacent, or second coil. A varying current in the first or primary winding or coil creates a varying magnetic flux in a core and thereby a varying magnetic field in a second winding or coil. The varying magnetic field induces an electromotive force in the secondary coil thereby inducing the electrical output voltage. Such electrical transformers perform many power manipulation processes in many industrial, commercial, and residential applications.
Desired operation of such transformers relies heavily on proper operation of the respective windings associated with the primary and second coils. Temperature deviations and thermal localizations associated with either of the primary or secondary windings can adversely affect the ability of the respective coils to conduct the input and/or output electrical power associated with the respective coil. Left unaddressed, such deviations can result in failure of the respective coil and/or ultimate failure of the transformer. Due at least in part to the voltage potentials common in such transformers, indirect measurement methods are commonly used to calculate winding temperatures as well as assessing and determining winding temperature hot-spots. Exemplary equations associated with determining winding and winding hot-spot temperatures can be found in IEEE standards C57.91, C57-119, IEEE-1538, and IEC 60076. Such equations utilize various variables associated with operation of a transformer such as one or both of a top oil temperature, a bottom oil temperature, and power transfer design parameters—such as winding ratios; conductor sizes, etc. to determine the target winding and winding hot-spot temperatures associated with intended operation of specific transformer configurations. However, the temperature of the transformer winding is not uniform and the hottest section of the winding is commonly called the winding hot spot.
An accurate model of the transformer's thermal performance is needed to simulate how the transformer will respond in power applications during operation of the transformer. As part of acceptance testing on new units, the temperature of the respective windings is measured to demonstrate that the average winding temperature will not exceed the acceptable limits as defined by various industry standards associated with the intended use of the transformer. The accuracy of the temperature calculations varies as a function of the accuracy of the information provided as well as a correlation between the ability of the mathematical model to simulate the thermal performance of a particular transformer.
In addition to the mathematical modeling discussed above, more advanced transformer monitoring systems can also calculate intermediate oil temperatures at locations in the transformer based on intended transformer operating conditions along with transformer design parameters. The winding hot-spot temperature of a transformer is an important value to monitor in order to safely operate and manage operation of a given transformer. Additionally, transformer life depends on the life of the insulating material in the transformer, and the life of the insulating material depends on the temperatures to which it is exposed.
When designing new transformers, engineers utilize theoretical parameters to calculate loss data and model the thermal performance of the transformer. The various parameters cannot be proved until the transformer is tested. The equations used to determine the winding and winding hot-spot temperatures can be found in the IEEE standards C57.91, C57-119, IEEE-1538 and IEC 60076-7. These equations utilize top oil temperature or bottom oil temperature along with other transformer design parameters to determine the winding and winding hot-spot temperatures. The accuracy of these calculated temperatures varies depending on the accuracy of the information provided and how well the model simulates the thermal performance of the transformer.
In addition to calculating the winding and winding hot spot temperatures, advanced transformer monitoring systems also calculate oil temperatures in various locations in the transformer. Similar to calculating winding and winding hot spot temperatures, transformer oil temperature calculations are based on environmental and transformer operating conditions along with the transformer design parameters as expressed as equation variables. Comparing the measured oil temperatures to the theoretical calculated oil temperatures can provide confirmation of the equation variables and creates a more accurate model of the thermal performance of the transformer.
Alternatively, direct methods of winding temperature and winding hot-spot measurement can be employed to compare the measured temperatures to the calculated values. Direct winding temperature measurement also provides a confirmation of the equation variables and helps create a more accurate model of the transformer.
The present invention utilizes advanced monitoring systems to automate both confirmation of the transformer parameters as well as the equation variables. The optimization software can automatically adjust the transformer parameters and equation variables to improve the model results such that the mathematical model more accurately reflects actual operation of the transformer. Alternatively, a transformer monitoring system according to the present invention can request an operator recommendation as to allowing or denying revisions to the model variables to improve the model results. As another alternative, the monitoring system can be configured to automatically manipulate one or more of the variables associated with the model equations in response to one or more monitored parameters, thereby automatically “tuning” the control parameters to create a more accurate model and thereby increasing the ability to predict performance of both existing and new power transformer systems.
One aspect of the invention contemplates a power transformer control system that includes a processor that is connected to a random access memory device and configured to be connected to a transformer. At least one sensor is connected to the processor and configured to detect a value, such as temperature or electrical parameters, associated with operation of the transformer. At least one mathematical model associated with operation of the transformer is stored on the random access memory device. The processor is configured to solve at least one mathematical model and compare a calculated value achieved by solving the at least one mathematical model to the detected value. The processor is configured to manipulate at least one parameter in the mathematical model if the calculated value and the detected value are beyond an acceptable range or desired respective limit. The processor can be configured to automatically manipulate the mathematical model or request user confirmation of the desired manipulation.
Another aspect of the invention contemplates a method for assessing operation of a power transformer. The method includes calculating theoretical winding temperatures of a transformer and measuring actual winding temperatures associated with operation of the transformer. The calculated theoretical winding temperature and the measured winding temperatures are then compared and a difference, if any, is determined between a calculated winding temperature and a measured winding temperature. A change, if any, is calculated for at least one of a hot spot factor, a winding exponent, and a winding time constant so that the calculated theoretical winding temperature approaches the measured winding temperature to improve a mathematical model associated with determining the thermal performance of the transformer.
Another aspect of the invention contemplates a method for calculating the thermal characteristics in a power transformer. A theoretical oil temperature associated with operation of the power transformer is calculated and actual oil temperatures are measured during operation of the power transformer. A difference between the calculated and the measured oil temperatures and a temperature time error is calculated between a calculated and a measured time of minimum and/or maximum oil temperatures is reduced by changes to at least one of oil rise values, oil exponents, oil time constants, and equation variables associated with calculating the theoretical oil temperature to get the at least one of the calculated error and the temperature time error to approach zero.
Another aspect of the invention contemplates a method for comparing the time at which a temperature will occur based on the mathematical model versus the time at which a measured temperature occurs in a power transformer. The difference between the time of the calculated and measured temperatures is reduced by changes to at least one of oil rise values, oil exponents, oil time constants, and equation variables associated with calculating the theoretical oil temperature.
These and various other aspects, features, and advantages of the present invention will be made apparent from the following detailed description and the drawings.